Towards a robust estimate of the merger rate evolution using near-IR photometry

Abstract

We use a combination of deep, high angular resolution imaging data from the CDFS (HST/ACS GOODS survey) and ground based near-IR $K_s$ images to derive the evolution of the galaxy major merger rate in the redshift range $0.2 \leq z \leq 1.2$. We select galaxies on the sole basis of their J-band rest-frame, absolute magnitude, which is a good tracer of the stellar mass. We find steep evolution with redshift, with the merger rate $\propto (1+z)^{3.43\pm0.49}$ for optically selected pairs, and $\propto (1+z)^{2.18\pm0.18}$ for pairs selected in the near-IR. Our result is unlikely to be affected by luminosity evolution which is relatively modest when using rest-frame J band selection. The apparently more rapid evolution that we find in the visible is likely caused by biases relating to incompleteness and spatial resolution affecting the ground based near IR photometry, underestimating pair counts at higher redshifts in the near-IR. The major merger rate was $\sim$5.6 times higher at $z\sim1.2$ than at the current epoch. Overall 41%$\times$($0.5\gyr$/$τ$) of all galaxies with $M_J\leq-19.5$ have undergone a major merger in the last $\sim8 \gyr$, where $τ$ is the merger timescale. Interestingly, we find no effect on the derived major merger rate due to the presence of the large scale structure at $z=0.735$ in the CDFS.